The present invention relates to a rapid thermal heating apparatus for heating substrates and, more particularly, to a heating apparatus for heating semiconductor substrates, including, for example, silicon wafers.
In semiconductor fabrication, a semiconductor substrate is heated during various temperature activated processes, for example, during film deposition, oxide growth, and etching.
When heating the substrate, it is desirable to heat the substrate uniformly--in other words, heat the substrate so that all the regions of the substrate are heated to the same temperature. This temperature uniformity in the substrate provides uniform process variables on the substrate. For instance in film deposition, if the temperature in one region of the substrate varies from another region, the thickness of the deposition in these regions may not be equal. Moreover, the adhesion of the deposition to the substrate may vary as well. Furthermore, if the temperature in one region of the substrate is higher or lower than the temperature in another region of the substrate, a temperature gradient within the substrate material is formed. This temperature gradient produces thermal moments in the substrate, which in turn induce radial local thermal stresses in the substrate. These local thermal stresses can reduce the substrate's strength and, furthermore, damage the substrate.
Conventional methods of heating a semiconductor substrate include indirect heating and heating direct methods. Indirect heating methods use resistive wire elements to heat the substrate. The resistive wires are typically adhered to or imbedded into a platform that supports the substrate. When the resistive elements are coupled to a power source and supplied with electrical current, the resistive elements heat the platform, which in turn heats the substrate. However, the amount of heat that the platform can deliver to the substrate is limited by the thermal diffusivity of the platform material. Furthermore, the heat-up rate of the substrate is limited to a temperature that will preserve the integrity of the adhesion of the resistive element to the platform material. This temperature typically does not provide the heat-up rate that is desired in semiconductor fabrication in which a rapid heat-up rate is preferred to minimize the total substrate processing heating budget.
Direct heating methods include the use of infrared lamps. Infrared lamps generally provide for a rapid heat-up rate and also achieve high substrate temperatures without any direct contact of the substrate. The drawbacks to the infrared heating technologies have been the lack of control of the temperature uniformity in the substrate and the significant power loss that occurs due to the fact that infrared lamps are cylindrical radiators. In a conventional direct heating apparatus, a semiconductor substrate, such as a semiconductor wafer, is disposed in a chamber and heated by radiation from a plurality of infrared linear lamps, for example T3 lamps commercially available from GTE Sylvania, which are approximately eight to twelve inches long. These lamps are known in the industry as Tungsten-Halogen lamps because their resistive lamp element is made of tungsten and the gas that fills the quartz lamp tube contains a concentration of a complex halogen gas mixture. These lamps typically emit a gray body type of spectral emission and, at full voltage, can peak at about 0.95 micron light wavelength--a wavelength that is almost totally absorbed by silicon.
The linear lamps produce a "flood" type rapid thermal heating process. Heretofore these "flood" type heating processes have not provided adequate control of the temperature profile in the substrate because the different regions of the substrate typically have different energy absorption or emissivity characteristics. In general, the regions close to the substrate's perimeter cool faster than the central regions of the substrate. For example, if a "flood" heating source is used to heat a semiconductor substrate during a rapid thermal processing cycle in which the thermally isolated substrate may be ramped to a temperature on the order of 10.degree. to 300.degree. C./sec., the peripheral regions of the substrate will maintain a different temperature than the central regions because the peripheral regions can accept radiant energy from a wider field of view and emit radiant energy to wider field of view. When constant uniform power is used to heat a substrate, the peripheral portions will be cooler than the central portions of the substrate during the ramp-up and cool-down portions of the heating cycle. As described above, these temperature gradients in the semiconductor substrate induce radial thermal stresses in the semiconductor substrate. These radial thermal stresses are generally not acceptable in many processes, especially in high temperature processes in which the crystalline integrity of the semiconductor substrate can be substantially compromised. Temperature differentials between the center and the edge as small as 5.degree. C. can induce warpage, defect generation, and crystal slip in the substrate. Although a bank of lamps have been used and modified to compensate for center to edge temperature differences during either the ramp-up or ramp-down phases of the heating cycles by using reflectors or shades, these configurations have not succeeded in providing the temperature uniformity that is preferred in these various temperature processes for semiconductor substrates.
Other types of T3 type lamps have also been used. These tend to be of the shorter variety, typically in the two to five inches in length and which usually have screw in type connectors. These lamps are usually inserted in a highly reflective base, such as in a flashlight reflector, but generally require a cooling system. Such heating systems offer an advantage to linear lamps since they can be arranged into a number of radial arrays with varying power inputs and, thus, provide better control of the substrate temperature uniformity. In general, applications using the shorter T3 type lamps usually require higher power densities per lamp and a higher number of lamps to effect the same power input. As a result, they tend to be less efficient heating systems.
The current technical evolution of these methods has focused on the development of reflector technologies that have been of complex reflectivity designs, which have been complicated by the need to cool the reflectors. For example in U.S. Pat. No. 5,487,127 to Gronet et al., a plurality of these shorter lamps are housed in light pipes that are inserted in a water-cooled quartz window assembly. The light pipes are independently controlled heat sources, each providing energy to a predetermined area of the substrate or wafer. Sensors for sensing the temperature of the area of the substrate heated by the heating apparatus provide feedback to control of the heat sources to maintain uniform temperature across the substrate. While the Gronet heating apparatus provides a more uniform application of heat energy to the substrate, the apparatus is generally less efficient because of the wasted heat energy and is more complicated, requiring sophisticated control circuitry.